As production processes of polymers, emulsion polymerization, suspension polymerization, solution polymerization and bulk polymerization are widely known. Depending on the polymerization process, the resulting polymer somewhat differs in properties. An appropriate polymerization process is, therefore, chosen and adopted in accordance with the desired polymer.
In particular, solution polymerization and bulk polymerization have found wide-spread utility, as they are resource saving and energy saving and they also permit easy solution of pollution problems by adopting them as closed processes.
In each of solution polymerization and bulk polymerization, however, dissolution of a polymer in a solvent results in a homogeneous phase system so that the viscosity of the polymerization mixture increases as the polymerization progresses. Another problem is also involved in that, as the production scale of a polymer increases, the heat removal area decreases inversely to the increase of the reactor volume.
In general, however, it is difficult to remove the heat evolved in a polymerization reaction from the polymerization mixture the viscosity of which is high. In addition, the reactor is brought into such a state that a zone which does not move for a long time, in other words, a zone of so-called extraordinary stagnation tends to occur in the reactor. When a zone of extraordinary stagnation occurs, the polymer formed in this zone is susceptible to deterioration or gelling, and may adhere the inside of the reactor. Mixing of such a polymer in a normal polymer results in a substantial impairment to the quality of the resulting polymer.
To overcome this problem, a variety of approaches have been proposed to avoid occurrence of a zone of extraordinary stagnation. One of such approaches is to complete polymerization without raising the final polymerization degree, that is, while the viscosity of the polymerization mixture is still low. According to this approach, the polymerization mixture under processing is low in viscosity so that a zone of extraordinary stagnation hardly occurs. This approach, however, has developed a new problem that the operation rate of the reactor is reduced.
Another approach is to apply a shearing stress to a polymerization mixture such that a shear rate in the vicinity of a heat transfer surface is increased to heighten the heat removing ability and also to prevent occurrence of a zone of extraordinary stagnation. This approach, however, is accompanied by a problem in that, when a screw-type agitating element is employed, vigorous mixing takes place in the reactor and the residence time distribution of the polymerization mixture in the reactor becomes broader with the agitating power.
To overcome the above-described problems, reactors have been proposed as polymerization reactors for processing fluid of increased viscosity, which have a narrow residence time distribution, that is, high piston flow characteristics and permit even removal of reaction heat.
These reactors include, for example, the reactor disclosed in U.S. Pat. No. 2,727,884 and the reactor disclosed in JP-A-04335001. The former reactor is equipped with plural heat exchanger tubes and agitating elements, which are combined in multiple stages. The latter reactor has agitating elements arranged at intervals in plural stages and passages arranged for a heat transfer medium between the agitating elements, and is also provided with a cylindrical heat transfer element arranged surrounding a periphery of a drive shaft.
The reactor disclosed in U.S. Pat. No. 2,727,884 tends to develop grid-like channeling in the flowing direction of a polymer and a zone of extraordinary stagnation when agitation is insufficient. As large clearances exist between the agitating elements and the heat exchanger tubes corresponding thereto, the reactor involves another problem in that the surface renewal rate on and along the surface of each heat exchanger tube is low, resulting in adhesion of the resulting polymer, blocking with the resulting polymer, a reduction in heat transfer coefficient or the like.
When agitation is conducted in an attempt to apply a shearing stress in order to increase the surface renewal rate on the surface of each heat exchanger tube, the agitating element is caused to vibrate in the direction of lift by so-called Karman's vortices produced from the agitating element, and by resonance, is brought into contact with the heat exchanger tube, leading to a stress fracture. Especially when the distances between the heat exchanger tubes and the agitating elements are small, such vibrations pronouncedly occur on the agitating elements.
As such vibrations of the agitating elements also occur in an initial stage of polymerization, at the time of an initiation and termination of polymerization, and in some instances, during washing of a reactor with a solvent or the like, no sufficient shearing stress can be applied in such a reactor in the initial stage of polymerization, at the time of the initiation and termination of polymerization, and in some instances, during washing of the reactor with the solvent or the like. As a consequence, the controllability of the polymerization reaction is reduced in the initial stage of the polymerization or at the time of the initiation or termination of the polymerization, and upon washing with the solvent or the like, the washing time becomes longer.
The reactor disclosed in JP-A-04335001, on the other hand, has the passages for the heat transfer medium, which are arranged between the agitating elements disposed in plural stages. Channeling of a polymer, therefore, occurs through the cylindrical heat transfer element arranged surrounding the periphery of the drive shaft or, when the flow velocity of the polymer passing through the heat transfer element is low, the surface renewal rate on the heat transfer element is low. As a consequence, this process involves a problem such as adhesion of the polymer, blocking of the heat transfer element or a reduction in heat transfer coefficient.
To overcome the above-described problems of the conventional art, the present inventors proposed in JP-A-11106406 a reactor in which each agitating element has a natural frequency of 40 Hz or higher.
It has, however, been found that, in this proposal, the agitating elements each having a natural frequency lower than 40 Hz can still be usable without developing vibrations depending on the rotating speed of the agitating elements or the viscosity of a polymer solution and also that, in this reactor, even the agitating elements each having a natural frequency of 40 Hz or higher in contrast may develop vibrations when the viscosity of a polymerization mixture is low, when the rotating speed is high or when the distances between heat exchanger tubes and the heat exchanger tubes are small. Due to the limitation of the natural frequency of each agitating element to 40 Hz or higher, agitating elements each having an unnecessarily large thickness relative to its width or agitating elements each having a large diameter are used even under such operation conditions that vibrations are not developed even when each agitating element has a natural frequency lower than 40 Hz. As a consequence, the above reactor has also been found to involve another problem that the effective volume of the reactor is decreased.
There is a known phenomenon that, as the moving speed of an object through a fluid increases, Karman's vortices are produced and that, when a forced frequency in the direction of the resulting lift coincides with the natural frequency of the object, the object develops resonance to result in a significant increase in amplitude in the direction of the lift. Agitating elements also develop resonance and a similar phenomenon when the rotating speed increases and the shaking frequency in the direction of lift coincides with the natural frequency of the agitating elements. However, the rotating speed of each agitating element is low at a base portion thereof close to the drive shaft but is high at tips thereof. It has, therefore, been difficult to determine to which extent of rotating speed of the agitating elements would not develop vibrations and would be safe. As the shaking frequency also varies depending on the distance between heat exchanger tubes and their adjacent agitating elements and the viscosity of each polymer solution, no proposal has been made yet on designing of a safe reactor on the basis of a formula which reflects these conditions.